Field of the Invention
This invention relates to the fields of biochemistry, immunology, molecular biology and medicine. More specifically, the invention relates to immunoglobulins and fragments thereof with the ability to hydrolyze or bind to bacterial, viral and endogenous polypeptides and to their preparation and to methods of use thereof.
Description of the Related Art
Some immunoglobulins (Igs) have the ability to catalyze chemical reactions through the binding of an antigen, its chemical transformation and the conversion and release of one or more products. Transformation of the chemical structure of antigens by such Igs can induce permanent inactivation of antigens. A single catalytic Ig molecule can hydrolyze thousands of antigen molecules over its biological lifetime, which enhances the biological potency of the catalyst compared to a stoichiometrically binding antibody. Catalytic Igs, therefore, can be developed as potent therapeutic agents capable of removing harmful polypeptide and other classes of antigens. Some Igs contain catalytic sites in their variable (V) domains that have properties similar to the catalytic sites of conventional serine protease class of enzymes. Igs with esterolytic and proteolytic activity have been reported [1-5]. Similarly, some Igs hydrolyze nucleic acids [6,7].
An appreciation of the structural organization of Igs is helpful in understanding the scope of the present invention. A brief review of this aspect follows. Generally, Igs contain light (L) chain and heavy (H) chain subunits. The V domains of these subunits contain the antigen binding site (paratope). Contacts with conventional antigenic epitopes occur mainly at the complementarity determining regions (CDRs) and to a lesser extent the framework regions (FR) of the V domains. The human Ig repertoire, defined as the number of Igs with different antigen binding site structures is estimated at 1011-1012. V domain diversity is generated by the following processes: (a) inheritance of about 50 germline genes each encoding the V domains of L and H subunits; (b) combinatorial diversity brought about by assembly of different L and H chains within the quartenary structure of Igs; (c) junctional diversity generated during recombination of the V and joining (J) gene segments of the L chain, and the V, diversity (D) and J gene segments of the H chain; and (d) rapid mutation occurring in the CDRs over the course of B cell clonal selection, a process entailing antigen binding to Igs expressed as components of the B cell receptor (BCR), and resulting in stimulation of division of the B cells expressing BCRs with the highest binding affinity. An additional level of diversity is offered by the use of different constant domains by Igs, that is, the μ, δ, γ, and a regions of the H chain and the κ and λ chains of the L chain. Early in the ontogeny of the immune response, Igs contain μ or δ constant regions. Later, isotype switching occurs, and the μ/δ regions are replaced by γ/α/ε in more differentiated Igs.
Various advances of technology in monoclonal and recombinant Ig techniques have accelerated the identification, selection and purification of catalytic Ig species. One approach to generating catalytic Igs involves immunizing an animal with a stable analog of the transition state of the reaction to be catalyzed and screening for Igs that bind more strongly to the transition state analog than to the corresponding substrate. The Igs, like enzymes, have a site that is complementary to the 3-D and ionic structure of the transition state analog. A large number of Igs synthesized in response to immunization with a transition state analog can bind the analog, but only a small minority will catalyze the reaction of interest. For example, only one catalytic Ig may be found for every 100-1,000 Igs screened. Another major challenge is that the catalytic activity of the Igs can be low compared to the naturally occurring enzymes, usually by a factor of 103 or more.
To be medically useful, the catalytic Ig must also be specific for the desired target antigen. While promiscuous catalytic Igs capable of hydrolyzing polypeptides are common [5,8-10], specific catalytic Igs directed to medically important target proteins are rare. Advances in the field of catalytic Igs, therefore, are dependent on identifying Igs that have high level catalytic activity and the correct epitope specificity enabling specific catalysis directed against the target antigen. The foregoing problems have been recognized for many years. Numerous solutions have been proposed, but none adequately address the problems that must be solved in isolating Igs that have the ability to specifically catalyze medically important biochemical reactions. Renewable and homogeneous sources of well-characterized catalytic Igs are needed for medical applications. A brief review of Ig technologies that may be useful in isolating catalytic Igs follows.
Traditional methods to clone Igs from humans consist of immortalizing lymphocytes derived from peripheral blood (or lymphoid tissues obtained by surgery), for example by transformation with Epstein Barr virus followed by fusion with a myeloma cell lines. The resultant hybridoma cell lines are screened for production of the desired Abs, for example by measuring the binding to a specific antigen by ELISA.
Methods are also available to clone the expressed Ig V domain repertoires in the form of libraries displayed on a suitable surface. The Ab fragments can be cloned as Fab fragments, single chain Fv (scFv) fragments or the L chain subunits. Fab and scFv constructs usually reproduce faithfully the binding activity of full-length Abs (e.g., [11]). Previous reports have documented the antigen binding activity of L chain subunit independent of its H chain partner, albeit at reduced strengths compared to native Abs [12,13]. The V domains of the scFv fragments are usually linked by flexible peptide linkers. Cloning of V domain repertoires is usually accomplished by recovering mRNA from lymphocytes and amplification by the reverse transcriptase-polymerase chain reaction. Mixtures of primers are employed to capture as large a proportion of the expressed repertoire as possible. The primers anneal to comparatively conserved FR1 and FR4 nucleotide stretches located at the 5′ and 3′ ends of the V domains, respectively, allowing amplification of V domains belonging diverse V gene families. To obtain expressible scFv constructs, the VL and VH domains are cloned into a suitable vector containing a short flexible peptide and an inducible promoter. Peptide tags such as the his6 tag are incorporated into the protein to enable rapid purification by metal affinity chromatography. The length and constitution of the peptide linker is an important variable in ensuring the appropriate intramolecular VL-VH interactions.
The next task is to isolate the minority of individual antigen combining sites with the desired antigen recognition characteristics. This can be accomplished using display technologies [14]. Vectors permitting display of recombinant proteins on the surfaces of phages, retroviruses, bacteria and yeast have been developed. For example, fusion proteins composed of Ig fragments linked to a phage coat protein are expressed from phagemid or phage vectors in bacteria. The recombinant phages display Ig fragments on their surface. The packaged phages contain single stranded DNA encoding the Ig fusion protein. Fractionation of phages based on binding to immobilized antigen yields, therefore, the VL/VH genes of Abs with the desired specificity. Phagemid vectors are useful because a codon at the junction of the Ab and phage coat protein genes is read as a sense codon by bacteria employed to package phages and as a stop codon by bacteria employed to obtain soluble Ab fragments free of the phage coat protein sequence.
Immunotherapeutic agents should preferably have a long half-life to avoid repeated infusions. The half-life of full-length IgG in human circulation is 2-3 weeks, compared to half-lives on the order of minutes for scFv constructs and free Ig L chain subunits. Therefore, various strategies have been developed to increase the stability of Ig fragments in vivo. Examples are the inclusion of a polyethylene glycol molecule at the Ig fragment terminus [15,16] or linkage to the constant region of Igs (Fc) [17]. The V domains can also be routinely recloned in vectors containing the constant domains of heavy and light chains. Expression of these vectors in suitable mammalian cells yields full-length Igs with increased half-life in vivo [18].
The properties of the target antigen are important in the success of medical applications of catalytic Igs. The targeted antigen can be chosen for Ig targeting based on the principles. First, the antigen should fulfill a pathogenic role. For example, the targeted antigen may interfere in some essential endogenous cellular or metabolic function. Alternatively, in the case of microbial antigens, the antigen should be important for growth of the microbe or it may be important in diverting host immune responses away from protection against the microbe. Second, removal of the antigen by Igs should not be associated with a deleterious side effect. This is particularly important in targeting of an endogenous antigen by Igs, e.g., amyloid β peptide in Alzheimer disease, as most endogenous antigens fulfill useful biological functions. In the case of microbial antigens, the danger of cross-reaction with endogenous antigens should be minimized. Immune complexes of antigens with Igs containing Fc regions have the potential of reacting with Fc receptors expressed on inflammatory cells and inducing undesirable inflammatory reactions. This danger is minimized if the Ig has catalytic activity, as the longevity of the immune complexes is reduced due to antigen chemical transformation and product release.
Examples of antigens suitable for Ig targeting are presented below.
Amyloid β Peptide (Aβ)
A β is the target of conventional non-catalytic Igs in ongoing clinical trials for the treatment of Alzheimer disease. A monoclonal IgG [19] and pooled polyclonal IgG from healthy humans [20] are under trial. The rationale for Aβ targeting is as follows. In 1907 that the first pathological lesion associated with dementia, the cerebral plaque, was reported by Alzheimer [21]. The cerebral lesion was called an “amyloid” plaque because iodine, which stained the cerebral plaque, also stains starch. The true chemical composition of the “amyloid” plaque was elusive until 1984 when Glenner and Wang discovered a means to solubilize the cerebral plaques in Alzheimer's disease (AD) and showed that they are composed principally of peptides Aβ1-40 and Aβ1-42. These peptides are identical but for the two additional amino acids, Ile and Ala, at the C-terminus of Aβ1-42 [22]. Both peptides are derived by proteolytic processing of the larger amyloid precursor protein (APP), which is composed of 770 amino acids and has the characteristics of a transmembrane protein [23]. APP is cleaved by β and γ secretases, releasing the Aβ peptides [24,25]. The role of Aβ peptides in the pathogenesis of AD is supported by findings that: (a) Familial AD is associated with mutations in the APP gene or the secretase genes; (b) Transgenic mice, expressing mutant human, APP genes develop an age-associated increase in cerebral Aβ peptides and amyloid plaques as well as cognitive decline [26,27]; (c) Mutant, human APP-tg mice that do not process APP to Aβ show no cognitive decline, suggesting that increased Aβ peptides and not the mutant APP is responsible for cognitive decline [28,29]; and (d) Synthetic Aβ peptides and their oligomers are neurotoxic in vitro [30,31].
HIV gp120
Igs directed to the HIV coat glycoprotein gp120 have been under consideration for immunotherapy of HIV infection. A key step in HIV infection is the binding of gp120 to host cell CD4 receptors. Additionally, gp120 plays a significant role in viral propagation and demonstrates a toxic effect on cells that are not infected with HIV [32,33]. gp120 is toxic for neurons, it facilitates lyses of lympocytes by an antibody-dependent mechanism, and it increases the binding of complement components to cells [34-39]. It has been shown that monoclonal Igs can bind the CD4 binding site (e.g., [40,41]). However, gp120 expresses many antigenic epitopes, and the immunodominant epitopes are located in the variable regions of gp120. Igs to the immunodominant epitopes do not neutralize diverse strains with varying sequence of the variable gp120 regions. Igs to the conserved gp120 sequences are necessary for therapy of HIV infection. Such Igs can also be used as topical microbicides to prevent vaginal and rectal transmission of HIV via sexual intercourse. gp120 contains a B cell superantigenic epitopes, defined as an antigenic epitope to which Igs are present in the preimmune repertoire without the requirement of adaptive immune specialization [42]. Residues 421-433 of this epitope are also important in CD4 host receptor binding [43,44]. Igs to the superantigenic epitope hold the potential of neutralizing HIV broadly. However, superantigens are thought to be recognized mainly by conserved regions of Ig V domains, including the conserved regions of the FRs and CDRs. Adaptive improvement of the superantigen recognition function appears to be difficult. No monoclonal or polyclonal Igs with the ability to neutralize the entire range of HIV strains belonging to various clades are available.
Staphylococcus aureus 
This bacterium is an opportunistic pathogen that colonizes the skin (primarily the anterior nasal vestibule) of approximately 30-50% (with 20-30% persistently colonized) of the population without causing clinical disease symptoms [45,46]. Persistently colonized individuals are designated ‘carriers’. Certain antibiotics are available for the treatment of S. aureus-caused disease, but the number of antibiotic-resistant strains is increasing rapidly. Host immunological factors play a role in determining susceptibility to initial colonization and development of S. aureus disease. However, certain virulence factors produced by S. aureus can downregulate host immune defenses profoundly, helping the bacterium colonize various anatomic sites and cause serious disease. The protective effect of Igs directed against S. aureus antigens has been reported in several experimental studies 147-491. Persistent antigenic exposure in S. aureus carriers can be hypothesized to induce adaptive synthesis of protective Igs. An insufficient adaptive response may be a predisposing factor in progression of infection. As noted above, certain microbial antigens behave as B cell superantigens. If S. aureus proteins have superantigenic character, protective Igs to the bacterium may be produced spontaneously without prior infection. S. aureus produces a host of virulence factors important in bacterial adhesion, toxicity for host cells and modulation of the host immune system. Selected S. aureus suitable for targeting by Igs are listed in Table 1.
TABLE 1Example S. aureus proteins suitable for targeting by IgsS. aureus proteinsFunctionEfbImmune ImpairmentProtein AImmune EvasionMap19 (Eap)Immune ImpairmentClfA229-54Fibrinogen/Fibronectin AdhesinClfB201-542Fibrinogen/Cytokeratin AdhesinFnbpAFibronectin AdhesinCANCollagen AdhesinSdrE51-606Potential AdhsinAlpha toxinToxinLukFToxinLukSToxinHepatitis C Virus (HCV)
It is estimated that over 170 million people worldwide are infected with HCV [50]. HCV genotypes 1a, 1b, 2a and 2b are common in the United States. The mainstay of current therapy is a combination of interferon and ribavirin, which leads to clinical improvement in a subpopulation of patients with chronic HCV infection. Clearly, more strategies are needed for treatment and prevention of HCV infection [51]. The E2 coat protein expressed by HCV is thought to be essential for viral infection by virtue of its role in host cell binding [52]. E2 contains hypervariable regions and comparatively conserved regions. Igs to the hypervariable regions are frequent in infected individuals [53]. Conventional non-catalytic Igs to E2 may be important in control of virus infection [53]. Certain monoclonal Igs to E2 neutralize the virus and [54] are under consideration for therapy of HCV infection.
Thus, there is a recognized need in the art for improved immunoglobulins that hydrolyze or bind to bacterial, viral and endogenous polypeptides. More specifically, the prior art is deficient in immunoglobulins and monoclonal antibodies derived therefrom comprising one or two immunoglobulin variable chain domains with an enhanced antigen-defined catalytic or binding ability. The present invention fulfills this long-standing need and desire in the art.